In the liquid jet printing field (often termed the ink jet printing field in view of the common use of ink in the jet printers), a wide variety of apparatus is available for controlling the trajectory of liquid jets and the selection of liquid for printing.
Until now it has been generally accepted that accurate and reliable high resolution printing can only be obtained by printing with droplets of liquid and that the deflection and selection of these droplets should be effected after the droplet formation has taken place. However, there have been several previous attempts to control the trajectory, and hence the placement, of liquid streams by causing forces to act on a coherent, unbroken section of the stream issuing from an orifice. These prior art attempts have met with only limited success and most have suffered from one or more major disadvantages.
In most of the previous examples of stream deflection apparatus, the liquid stream is deflected by an electrostatic field and either the deflected or the undeflected part of the stream is interrupted before it impinges upon the printing substrate. The interruption has been effected either by a baffle or collector arrangement, or by the part of the stream to be collected being brought into contact with a convex deflector surface.
In the specification of U.S. Pat. No. 1,941,001 granted to Clarence W Hansell and assigned to Radio Corporation of America, an apparatus is described whereby an unbroken liquid stream is attracted by an electrode to which a high voltage has been applied, so that its deflected trajectory is interrupted by a baffle placed between the stream and the printing surface. In this case the stream is permitted to impinge upon the printing surface when in the undeflected trajectory and is interrupted (and thus is prevented from reaching the printing surface) when it is deflected. Since the transition of the stream from one trajectory to the other takes a finite time, the leading edge of the collector intercepts liquid in the transition region between the deflected and the undeflected region. This leads to a build up of liquid on the collector edge. This build-up reduces the selectivity of the collector, and leads to poor resolution of the printing and to fouling on the printed surface.
In another form of apparatus, described by N. E. Klein and W. H. Stewart in the specification of U.K. patent No 1,456,458, an air jet from a hollow tube is directed at a liquid stream to deflect an unbroken portion of the stream away from a direct trajectory to the substrate or surface being printed, into a trough or collector which intercepts the stream and prevents it reaching the printing surface. In this system the frequency response of the ON-OFF transitions of the stream are restricted by the switching speed of an electro-pneumatic valve which is used to direct the air current against the stream. This low speed of response directly translates to a lower resolution and quality of print than is possible with higher speed systems.
In yet another type of apparatus, which is described in the specification of U.S. Pat. No. 3,893,623 to Richard A Toupin (assigned to International Business Machines Corporation), a liquid stream is amplitude modulated to produce discrete droplets. A weir, placed at a critical location downstream and adjacent to the trajectory of the stream and droplets, intercepts selected droplets if the diameter of the periodic disturbance on the liquid stream is greater than the necessary value to clear the weir. Toupin's specification discloses the use of a curved surface to capture droplets at the droplet formation point in the liquid stream. However, the sloped collector surface (see FIG. 3A of that specification) is not designed for the high collection efficiency which may be obtained when using the coanda effect to capture liquid.
In an alternative method of jet printing, which uses electrodes placed in close proximity to a coherent unbroken stream and which is described in the specification of U.S. Pat. No. 4,384,296 to Peter A Torpey, the liquid stream is modulated to form discrete droplets and requires further deflection apparatus to properly define exact print positions for the droplets.
In a further disclosure relating to apparatus for steering fluid jets, namely the specification of British patent No. 2,041,831 to Graham Francis Stacy, a liquid jet is steered by causing it to come into contact with a convex curved surface. Contact between the liquid stream and the curved surface is effected by mechanical movement of either the convex surface or the jet body, or alternatively by frequency modulation of the jet. This technique has a number of disadvantages. It is very difficult to achieve the required close spacing of the deflecting curved surface relative to the liquid stream, and to simultaneously achieve the required relative displacement of the jet body and the convex surface. Also, no mechanism is described whereby the undeflected stream can be prevented from reaching the printing surface.